Jet Spouted Bed Type Reactor Device Having A Specific Profile For CVD

ABSTRACT

The invention relates to a jet spouted bed reactor, comprising a cylindrical area, a gas injection pipe at the base of the cylindrical area, and a transition area, connecting the upper end of the pipe to the base of the cylindrical area, this transition area having a convex profile in a plane extending through the axis (YY′) of flow of a fluid in the pipe.

TECHNICAL FIELD AND PRIOR ART

Within the scope of methods implementing a deposition on powders orparticles by techniques of the type known as CVD (Chemical VapourDeposition), it is sometimes necessary, depending on the characteristicsof the granular material on which it is wished to carry out thedeposition, to implement a fluidisation by jet spouted bed. Thisimplementation enables an efficient mixing (and thus a homogeneousdeposition) of the bed of powder despite the difficulty of fluidisingthe latter. The present invention proposes a specific conception of jetspouted bed reactor enabling an improvement of the control of thedeposition conditions for cases inducing a significant modification ofthe characteristics of the bed by the very fact of this deposition. Thisis typically the case of jet spouted bed type reactors dedicated to theelaboration of fuel particles that can be used (TRISO particles) for HTR(High Temperature Reactor) type reactors.

It will be recalled firstly that jet spouted beds induce the formationof a fountain above a bed of particles. This technique is thus verydifferent to that of the fluidised bed, which only causes a relativelyhomogeneous mixing of a bed of particles. Jet spouted bed type reactors,on the one hand, and fluidised bed type reactors, on the other hand,thus do not at all have the same hydrodynamics, are not optimised in thesame way and the optimisation of one of the 2 types does not enable theoptimisation of the other type.

The first jet spouted beds were described by Mathur and Gishler (Mathur,K. B., Gishler, P. E., A.1.Ch.E. J. 1, 157 (1955)).

Fluidised bed reactors are described for example in Techniques del'Ingénieur (Fluidised bed reactor calculation, J4100, 10 Mar. 1992Khalil Shakourzadeh) and one of the first patents describing this typeof reactor is for example the German patent DE 437,970 (1922) of F.Winkler

The fluidisation of granular charges (known as bed) is a verysatisfactory method for bringing into contact a gas and these chargeswithin the scope of industrial methods in very multiple applications(drying, coating, catalytic reaction, etc.).

This phenomenon of fluidisation is not however easy for all types ofparticles, as has been shown by Geldart in his classification of powders(Geldart D. (1973), Powder Technology. Vol. 7, p 285).

The graph of FIG. 8 resumes this classification: one finds, on theY-axis, the difference in specific mass between the fluidisation gas andthe powder to be fluidised and, on the X-axis, the average diameter ofthe powder considered.

One may distinguish in this graph powders:

-   -   of the area A, which correspond to powders relatively easy to        fluidise, i.e. with a homogeneous fluidisation (without large        bubbles in the fluidised bed),    -   of the area B, which is that of powders which can be fully        fluidised,    -   of the area C, which corresponds to cohesive powders, very        difficult to fluidise: these powders do not come into movement        in a homogeneous manner in the bed, which results in important        variations of the head loss, in the bed, for flow rates        nevertheless very similar or equivalent, according to the        fluidisation tests,    -   of the area D, which is that of relatively dense powders, which        can be fluidised in the favoured conditions of a jet spouted        bed.

For particles known as type D (particles of large diameters and highdensities) in this classification, the fluidisation is conventionallyimplemented by means of a jet spouted bed reactor. In this type of bed,the gas enabling the fluidisation of the charge is introduced in thelower part, leading to a periodic movement and the formation of afountain, hence the name of jet spouted bed.

Different types of bed are known.

Jet spouted beds are known with a right angle profile, the simplestembodiment that exists, because the injection of gas is assured by aninjector at the level of a cylindrical section. This type of profile,represented schematically in FIG. 1A, is not optimised from the point ofview of the fluidisation of the granular charge, since it leads toimportant dead volumes, in other words areas 1, 1′ in which the passagetimes of the particles are very long, compared to the average of theother areas of the bed. In this sense, the right angle configurationtype is virtually not used industrially.

Conical profile jet spouted beds, of the type represented schematicallyin FIG. 1B, are conventionally used for the implementation of powdersthat are difficult to fluidise, in the sense of Geldart. Numerousexamples thereof are given in the prior art, for example in thedocuments CA 601607, U.S. Pat. No. 4,342,284, GB 1567256. This type ofjet spouted bed responds well to the need for the deposition methods, inso far as the characteristics of these charges (diameter and apparentspecific mass of the particles mainly), on account of the very fact ofthis deposition, are not significantly modified.

But, in the case where there is a constraint to implement a reduced massof charge (case of expensive material or which could lead to a risk ofcriticality, for example) important edge effects become manifest. Infact, it can then not be envisaged to reduce in an ill-considered mannerthe diameter of the reactor, because edge effects (influences of thewall on the flow of the powder) are not negligible up to a distance tothe wall substantially less than or equal to at least 10 times thediameter of the particles to be fluidised. Thus, it is not possible toreduce the reactor diameter too much without perturbing a highproportion of the flow of the powders. This type of bed is thus notoptimised, since the main part of the charge, when it is reduced, isthen situated in the conical part 2, of volume V_(o), between thesedimensions Z₀ and Z₁ of FIG. 1B (between the outlet orifice of the gasinjection pipe 18 and the lower part of the cylindrical portion 12),which makes the conditions, during deposition, not very stable; inparticular, the gas flow velocity through the particles is not constantduring the deposition, due to the change of passage section.

Concave profile jet spouted beds, an example of which is representedschematically in FIG. 1C, are used for generally important charges.Several industrial examples are to be noted as described in the documentEP 1752991. For the case of a forced limitation of the quantity ofmaterial, this type of profile is even more unfavourable, since itmaximises the volume V_(o), for a given transition height (Z₁-Z₀),defined as above, between the outlet orifice of the gas injection pipe18 and the lower part of the cylindrical portion 12.

In certain cases, the profiles of conventional jet spouted bedsdescribed above are made more complex. Thus, it is particularly possibleto implement moveable elements to aid fluidisation (case of documents EP1 550 502 or CN 10 183 06 27 for example) or to arrange beds in series(as in US 2009/0149620). The methods of injecting fluidisation gas canalso be modified to favour fluidisation (as described in RU2377487).These devices are optimised for a specific method of application, butare not very well suited for the case of an implementation of material,such as a nuclear material, not allowing frequentinterventions/maintenances during the implementation of solutions assophisticated and less robust than the simplified profiles describedpreviously.

Moreover, as for the cases described above, these profiles do not makeit possible to minimise the volume Vo, a relevant criterion particularlyin the case of an implementation of a limited quantity of material, asalready evoked above.

The problem is thus posed of being able to carry out one or severaldepositions, in particular by CVD technique, on a charge that isdifficult to fluidise, particularly in the sense given by Geldart, alsoleading to a high risk of clogging and/or plugging, particularly at thelevel of the injection of gas (in other words the dimension Z₀).

A powder may also be considered as difficult to fluidise if the minimalgas velocity, necessary for this fluidisation, is very high and if thephenomenon is not very reproducible or at least very sensitive to slightthermohydraulic variations or characteristics of the powders; one alsospeaks of not very easy fluidisation when the head loss brought about bya gaseous flow through the bed to be fluidised is fluctuating, despite aslight increase in the flow velocity of the gas through the bed ofpowder to be fluidised.

As an example, this is the case for the manufacture of particles knownas TRISO (i.e. with three layers) entering into the composition ofnuclear fuels for reactors known as HTR (High Temperature Reactor). Themass of uranium that can be handled to enable an easy implementation ofthe charge (i.e. without risk of criticality) is then often limited to350 or 600 g of uranium 235 (depending on the margins imposed by safetyrules and depending on the isotopic enrichment of uranium implemented).One then seeks to carry out multiple depositions in the same CVDreactor, which leads to, during each successive phase of deposition, amodification of the diameter and the apparent density of the particlesto be fluidised (the diameter is typically doubled during this type ofdeposition and the densities, on the other hand, may be reducedthreefold). In this case, this important variation of characteristics ofthe charge during deposition (and because of the very fact of thelatter) may be very disadvantageous because the conditions offluidisation of the charges are going to be, consequently, modified.This is all the more significant when the deposition takes place in thetransition volume known as V_(o) as defined above.

3 constraints need moreover to be taken into account:

-   -   the first is the fact that the charge on which the deposition        must be carried out is assumed, by virtue of said deposition, to        vary significantly in terms of apparent density and        granulometry,    -   the second resides in the fact that the charge on which the        deposition has to be carried out is necessarily of limited mass,        due to the fact that this charge is, for example, very expensive        and/or liable to lead to a risk of criticality,    -   the third resides in the desire to limit the risks of plugging        at the bed/emerging gas injector interface.

One thus seeks a specific profile of reactor making it possible,preferably at one and the same time, to:

-   -   limit the transition volume Vo (volume of the bed situated        between the dimensions Z₀ and Z₁ in the diagrams of FIGS. 1B and        1C),    -   and/or to minimise what are called dead areas of the bed and the        gaseous phase used for the deposition,    -   and/or to limit head losses while guaranteeing a sufficient        fluidisation with respect to the charge.

No known structure of jet spouted bed exists that responds to thiscomplex problem.

The present invention thus proposes resolving the problem posed.

It is posed particularly with great acuteness in the case of themanufacture of multilayer fuel particles used in HTR type nuclearreactors.

DESCRIPTION OF THE INVENTION

A jet spouted bed reactor is described herein, comprising a cylindricalarea, a single gas injection pipe at the base of the cylindrical area,and a transition area, connecting the upper end of the pipe to the baseof the cylindrical area, this transition area having a predominantlyconvex profile in a plane extending through the axis of flow of a fluidin the pipe.

The profile may have areas of convexity and areas of non-convexity; butthe areas of non-convexity are predominant, in other words that theirareas (or volumes if 3D is considered) as defined, compared to thehollowing out of a conical straight profile, are greater than those ofconvex areas.

One thus describes a new transition profile between the injectionsection of the gas of the jet spouted bed and the cylindrical terminalsection (or lower) of the reactor, which makes it possible to minimisethe volume of solid to be fluidised which is not comprised in thecylindrical section (volume Vo). This makes it possible to render thebed of powder to be coated less sensitive, during the deposition, tochanges of density ρ and of granulometry d and leads to a better controlof the deposition throughout the whole process.

As already explained above, the evolution of these parameters of densityρ and of granulometry d leads to an evolution of the depositionconditions and, thus, adjustments of the implementation conditions ofthe method, except if one is capable of accompanying this evolution tocontrol directly the deposition, which is very awkward and leads tobehavioural instabilities of the bed. But the thermohydraulic conditionsare not constant and are sometimes chaotic and it is preferable to havea jet spouted bed reactor profile that smoothes out as best as possiblethese fluctuations. The best thing is then to work in the cylindricalarea (i.e. above Vo) since it involves the most optimised shape, i.e.the least sensitive to the evolution of the diameters of the particlesand the apparent densities thereof vis-à-vis the deposition conditions.

The type of profile proposed in the present application enables a rapidbut efficient transition and particularly without creation of deadvolume, between the gas injection section, enabling the fluidisation byjet, and the cylindrical section of the deposition reactor.

Furthermore, this type of profile enables a minimisation of the risks ofplugging (or the risks of particle/wall/injector sticking) at the levelof the gas injector of the jet spouted bed.

In fact, in the case of a convex bed, the section of the passage, justafter the outlet of the gas injection pipe, is more restricted than inthe case of a concave or flat profile.

It ensues that the velocity of gas, at the outlet of the gas injectionpipe, is greater in the case of a bed with a convex profile than in thecase of a bed with a concave or conical profile. Thus, for a same flowrate of injected gas, the gas flow velocity as well as the velocity ofthe particles at the level of the gas injector will be greater with aconvex profile. In the case of the example of particles known as TRISO,the injected gas contains particularly the carbon precursor to bedeposited on the particles.

The angle β between the axis (YY′) of flow of a fluid in the pipe andthe tangent to the convex profile, at the point where it joins the pipe,is preferably at least equal to 5°, and, preferably, less than 45°, andeven more preferentially, comprised between 10° and 35°.

The convex profile may have a point of inflexion; in this case, oneforms an angle θ₁ between, on the one hand, an axis perpendicular to thedirection of flow of a fluid and extending through the base of thecylindrical part and, on the other hand, the tangent to the point ofinflexion I of the convex profile, preferentially greater than 10°and/or greater than the angle of spontaneous flow of the powderconstituting the bed to be fluidised on an inclined plane.

Preferably the angle θ₂ between, on the one hand, an axis perpendicularto the direction of flow of a fluid and extending through the base ofthe cylindrical part and, on the other hand, a straight line extendingthrough the end of the gas injection pipe and through the base of thecylindrical part, is less than 90° and/or greater than the angle ofspontaneous flow of the powder constituting the bed to be fluidised onan inclined plane.

A distance, between the end of the gas injection pipe and the base ofthe cylindrical section of the reactor, will preferably be chosengreater than 1/10 of the radius of the bed in this cylindrical section.

The convex profile may further be defined by a succession of N (N≧2)straight line segments (dn) of which none is vertical.

A method of fluidising particles of a bed of particles may comprise theimplementation of a jet spouted bed reactor as above. Advantageously,the particles, or at least a part of them, have a diameter greater thanor equal to 100 μm or to 500 μm, or close to one of these values, and/orthe difference in specific mass between the fluidisation gas and thepowder to be fluidised being greater than or equal to 0.1 g/cm³ or to0.5 g/cm³ or 1 g/cm³ or 5 g/cm³.

It is possible particularly to carry out a plurality of successivedepositions on said particles, for example with a gas of a first nature,then a gas of a second nature, different to the first.

For example, the particles comprise multilayer fuel particles used inHTR type nuclear reactors.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described, by way ofnon-limiting examples, while referring to the appended drawings, inwhich:

FIGS. 1A-1C are schematic views, in transversal section, of knownsystems,

FIG. 2A is a schematic view, in transversal section, of a jet spoutedbed with a new profile, according to the invention, comprising in thisexample a point of inflexion, for the gas injection section of a jetspouted bed and the cylindrical terminal section of a reactor,

FIG. 2B is a schematic view, in transversal section, of a variant of ajet spouted bed with a new profile, according to the invention,comprising an area of convexity and an area of non-convexity,

FIG. 3 is a detailed view, in transversal section, of a jet spouted bedwith a new profile for the gas injection section of a jet spouted bedand the cylindrical terminal section of a reactor,

FIGS. 4A and 4B are comparisons, for a same granular charge (in terms ofmass and physical characteristics), of the evolution of the head lossthrough a known bed with linear profile (FIG. 4A) and a bed with aninjection section with convex profile with a β angle=0 (FIG. 4B),

FIG. 4C represents the evolution of the head loss through a bed with aninjection section with convex profile, for a β angle value=10°,

FIG. 5 represents an injection section with convex profile, approachedby a set of straight lines segments,

FIG. 6 represents examples of profiles of an injection section withconvex profile,

FIGS. 7A and 7B are examples of embodiment of an injection section withconvex profile,

FIG. 8 schematically represents a known classification of powders.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

An example of a new profile of injection section is described withreference to FIG. 2A.

In this figure may be recognised schematically the structure of a part12, with cylindrical section, of a fluidised bed and the injection area14 in this cylindrical part.

As may be seen in this figure, this injection area 14 does not have aright angle, or conical, or concave profile, as described above withreference to FIGS. 1A-1C. Examples of dimensioning, and particularly ofthe height h₁ and the internal diameter d₂ (=2×r₂) of the cylindricalsection 12 are given hereafter. More generally:

-   -   the height h₁ is, preferably, greater than one tenth of the        radius r₂    -   and/or less than 5 times d₂.

The representations of FIGS. 2A and 2B must be taken as being withsymmetry of revolution around the vertical axis YY′, which merges withthe axis of the gas supply pipe 18.

The injection area 14 has, in a plane containing the axis YY′, a convexprofile, with the convexity turned towards the exterior of the device.By way of comparison, in dotted lines, a conical profile 16 isrepresented, which starts from the outlet of the gas supply pipe 18 andwhich joins the base or the lower end of the cylindrical part 12. It maybe noted that, generally speaking, the convex profile 14 is situated“above” the conical profile 16.

FIG. 2B represents a profile variant according to the invention. In thisfigure, identical references to those of FIG. 2A designate identical orcorresponding elements. It may be seen in this FIG. 2B that the profile14 may, in a plane containing the axis YY′, not only be convex, but thatit may have areas 14-1 of convexity (the convexity is turned towards theexterior of the device) and areas 14-2 of non-convexity (in other wordsnot having convexity turned towards the exterior of the device). Theareas of non-convexity are predominant, in other words the sum of thehatched surfaces S1 (or volumes if 3D is considered) comprised betweenthe profile of the convex areas 14-1 and the conical straight profile 16(already defined above) is strictly greater than the sum of the hatchedsurfaces S2 (or volumes if 3D is considered) comprised between theprofile of the non-convex areas 14-2 and the conical straight profile16.

In other words, the volume (hatched in the figure), defined between theplane P that passes through the lower end of the cylindrical part 12 andthe plane P′ that passes through the upper end of the gas supply pipe 18is, in the case of the convex, or predominantly convex, profile lessthan it is in the case of a conical profile, and, consequently, than itis in the case of a right angle profile or in the case of a concaveprofile.

The new profile proposed thus makes it possible to minimise the volumeof solid to be fluidised which is not comprised in the cylindricalsection (volume V₀), while enabling an efficient fluidisation. Thismakes it possible, particularly in the case of a powder intended to forma coating, to render the bed of powder less sensitive to changes ofdensity ρ and of granulometry d during the deposition and therebyenables a better control of the deposition throughout the whole process.

Examples of profiles reproducing the section 14 are given in FIG. 6.

Each of these profiles follows an equation:

R ₁=2.65+0.00533z ^(2;)

R ₂=2.62+0.176z-3.81×10⁻³ z ²+9.6×10⁻⁵ z ^(3;)

Unlike these two examples, a convex profile cannot always be expressedin the form of an equation.

It is also possible to form a convex, or predominantly convex, profileby a succession of straight line segments, as illustrated in FIG. 5.

The convex, or predominantly convex, profile is then formed by asuccession of N (N>1 or N>2 or N>5) straight line segments d_(n) (n=0,1, . . . N), each segment being comprised between two ends I_(n),I_(n+1;) I_(n) is thus the point of intersection between these straightlines segments d_(n) and d_(n−1) with the following expression of thestraight line d_(n) (n being ≧1):

${Y(n)} = {{\frac{y_{n} - y_{n - 1}}{x_{n} - x_{n - 1}} \cdot x} + y_{n} - {( \frac{y_{n} - y_{n - 1}}{x_{n} - x_{n - 1}} ) \cdot x_{n - 1}}}$

with y_(n) and x_(n) the coordinates of the point I_(n) in the referencepoint (x,y) as defined in FIG. 5, the Y axis being merged with theprojection, in the plane YX, of the lateral part of the cylindrical body12, and the X axis being perpendicular to the Y axis and extendingthrough the point B of intersection between the cylinder 12 and theconvex portion 14. The origin I₀ is situated at the intersection ofthese two axes.

In this representation, the angle α_(n) formed between the straight lined_(n) and the horizontal (the axis I₀X) is defined by the followingexpression

${\tan \mspace{14mu} \alpha_{n}} = {- \frac{y_{n} - y_{n - 1}}{x_{n} - x_{n - 1}}}$

As will be seen hereafter, it is preferable to adopt the followingconstraint on the angle a

(or α_(n)):

-   -   α>10° except for n=1    -   and α_(n)<80° for n=N (with N≧2)

The values of the parameters of the profiles may be optimised as afunction of the desired minimisation of the volume V₀.

This gain may be defined by the expression below, by considering a zerogain when the convex profile is superimposed on the conical profile (orprofile standard):

${Gain} = \frac{( {V_{0}^{P} - V_{0}^{Cone}} )}{V_{0}^{Cone}}$

where V^(P) ₀ and V^(Cone) ₀ represent, respectively, the volume V₀ forthe case of a profile (here: convex) P and for the case of a flatprofile, equivalent to a truncated cone.

Given that

V₀^(P) = π∫₀^(Z 0)f(x)²x

with f(x) being the function of the profile P considered as a functionof the coordinate x, it is then possible to estimate the gain in volumefor a given profile compared to that of a flat profile.

For the convex profiles R1 and R2 already mentioned and illustrated inFIG. 6, the gain

is close to 30%, which represents a significant value.

In a detailed manner, in FIG. 3, are represented a portion of thecylindrical area 12, the gas supply pipe 18, and the convex section 14that connects them, between a point A, situated in the upper part of thepipe 18, the point B, which corresponds to the intersection between thelower part of the cylindrical area 12 and the convex profile 14.

One defines, in a vertical plane, which contains the axis yy′ ofinjection of gases, the following angles, which are going to make itpossible to characterise a little more convex, or predominantly convex,profiles particularly adapted within the scope of the present invention:

-   -   θ₁ is the angle formed between:    -   a horizontal axis that extends through a point B of the lower        end section of the cylindrical portion 12;    -   and the tangent to the point of inflection I of the convex, or        predominantly convex, profile in the case where such a profile        has at least one zero second derivative point;    -   θ₂ is the angle formed between:    -   the horizontal axis that has been defined    -   and the tangent to the convex profile, at the point B;    -   θ₃ is the angle formed between the same horizontal axis and a        conical profile at the point A (point situated on the        circumference of the outlet of the gas injection pipe 18); this        conical profile is represented in dotted lines in FIG. 3 and        connects the point A to the point B;    -   β is the angle formed between the vertical axis YY′ and the        tangent to the convex profile, at the point A.

As regards the dimensions, there is, in this figure:

-   -   the radius r₁, which is the injection radius of the gas        injection pipe;    -   the radius r₂, radius of the fluidised bed in the cylindrical        part 12 of the jet spouted reactor; it corresponds substantially        to the radius of the cylindrical part, in a plane perpendicular        to the axis YY′;    -   the height h₁, or transition height between the dimension of the        gas injection (dimension Z₀), and that from which the section of        the bed is constant along the vertical axis (dimension Z₁).

The convex, or predominantly convex, profile of the reactor ispreferably chosen in the following manner, which favours thefluidisation of the solid charge.

In particular, the appearance of a vault or of an arch of solid materialmay be difficult to break if the profile retained is too convex,particularly if the value of the angle β (angle between the vertical andthe tangent to the profile of the reactor) is too small.

FIGS. 4A and 4B are comparisons, for a same granular charge (in terms ofmass and physical characteristics: monomodal powder with a density d=500μm and a specific mass ρ=5.9 g/cm³), of the evolution of the head lossthrough:

-   -   a bed with known linear profile (FIG. 4A), conical, with θ₃=60°;    -   and a bed with an injection section with convex profile with an        angle β=0 (FIG. 4B).

FIG. 4C represents the evolution of the head loss through a bed with aninjection section with convex profile, for a β angle value=10°, for apowder having the same characteristics as above.

In these 3 diagrams, the black triangles correspond to the case of anincreasing flow rate, and the crosses to the case of a decreasing flowrate.

In the case of a conical profile (FIG. 4A) one distinguishessuccessively a regime I₁, called “fixed bed”), a regime II₁, for whichthe lower part of the bed is in movement, a regime III₁ called irregularfountain, followed by a regular fountain regime IV₁.

In the case of a convex profile, with β=0 (case of a profile tangent tothe vertical), one has successively a regime I₂, known as “fixed bed”, aregime II₂, for which the lower part of the bed is in movement, a regimeIII₂ of surface eddies, a regime IV₂ of irregular piercings, and aregime V₂ called irregular and turbulent fountain. It has been able tobe shown that, for a dense charge (for d>6) and of granulometry close to500 μm, fluidisation is difficult, which may be seen in FIG. 4B. Thismay, in particular, result in an important increase (up to more than 100mbar for a bed height close to 70 mm) of the head loss as a function ofthe increase in the flow rate of injected gas (it is the regime I₂,known as “fixed bed”) comparatively to which may be noted with a morehollowed out or even conical profile (case of FIG. 4A).

Furthermore, the fountain generated by the injection of gas, in the caseof a convex profile, is more unstable. A minimal value of β exists toobtain a fluidisation considered satisfactory, in stable regime.

In FIG. 4C is represented the evolution of the head loss through a bedwith an injection section with convex profile, for a β angle value=10°.It may be seen that, as a function of the flow rate, the head loss isrelatively regular, whether the flow rate is increasing or decreasing.Moreover, the equivalent head loss, in the regime I₃ called fixed bed,is indeed less than for the case where β=0°, which denotes a betterbehaviour to fluidisation in the case of this profile β=10°, hence therecommendation of having a β angle at least equal to 10°.

In this diagram is indicated, successively, the region II₃ of the firstpiercing of the bed, the area III₃ of appearance of surface eddies, aregion IV₃ of irregular piercing, the appearance V₃ of bubbles at thesurface, then the formation, in VI₃, of a regime called “fountain”.

Preferably, for the parameters θ₁, θ₂, β, r₁, r₂ of a convex profile thefollowing values are chosen:

-   -   β: comprised between 5° and 45°, more preferably between 10° and        35°;    -   and/or θ₁ greater than the angle of spontaneous flow of the        powder constituting the bed to be fluidised on an inclined plane        (which describes the capacity of a powder to flow naturally; to        do so, a heap of this powder is formed on an initially        horizontal flat surface, then, the slope of this flat surface is        made to vary progressively according to a greater and greater        angle: the angle of spontaneous flow is that for which the        powder is going to flow naturally); for example θ₁≧10°;    -   and/or θ₂ greater than the angle of spontaneous flow of the        powder constituting the bed to be fluidised; θ₂≦90°;    -   and/or r₁:5 D (where D is the diameter of the particles to be        fluidised)≦r₁≦r₂/5;    -   and/or h₁: >r₂/10

An example of convex profile formed is represented in FIGS. 7A and 7B.

It may be seen that, in this example:

-   -   h₁=68.2 mm;    -   d₂=2×r₂=70 mm.

Moreover, in FIG. 7B are represented various dimensions of the convexprofile, in other words the distance, for various vertical positions,from the surface of this profile with respect to the axis YY′.

Thus it may be seen that, if one takes the origin 0 at the level of theoutlet of the gas injection pipe, the distance, with respect to the axisYY′, varies from several millimetres to around 20 mm, between a positionclose to the outlet of the gas injection pipe 18 and a position situatedat around 60 mm from this outlet.

In FIG. 7A are also represented the means, here threaded holes 30, 30′,which will make it possible to fix the cylindrical section 12 onto thebase 140 in which the convex profile 14 has been formed.

This profile has been able to be machined in CAFM, which makes itpossible to use an equation during a machining.

It has moreover been able to be tested.

It involves, in fact, the profile corresponding to equation R2, ofdegree 3, as already defined above.

An example of application relates to the manufacture of multilayer fuelparticles used in an HTR type nuclear reactor. This type of reactoroperates at high temperature, with a gaseous heat transfer fluid such ashelium, the moderator being the graphite constituting the fuel.

For the manufacture of TRISO particles constituting nuclear fuel, areactor device is implemented as has been described above (with thegeometric parameters of the example given previously in FIGS. 7 A and7B).

Successive depositions are carried out of pyrocarbon (internal PyC),then SiC, then again pyrocarbon (external PyC) on particles (known ascores) of UO₂ of, for example, 500 μm diameter.

The depositions are then successively carried out from acetylene andethylene (for depositions of pyrocarbon type) and methyltrichlorosilane(for the deposition of SiC).

The flow rate of gas is of the order of 10 Nl/min for all gases, thecarrier gas of the precursor (the source of pyrocarbon or SiC) beingargon.

The operating temperature of the CVD reactor is comprised between 1200and 1600° C., depending on the nature of the deposition to be carriedout.

1. Jet spouted bed reactor, comprising a cylindrical area, a gasinjection pipe at the base of the cylindrical area, and a transitionarea, connecting the upper end of the pipe to the base of thecylindrical area, this transition area having a convex, or predominantlyconvex, profile in a plane extending through the axis (YY′) of flow of afluid in the pipe.
 2. Jet spouted bed reactor according to claim 1, inwhich the angle β between the axis of flow of a fluid in the pipe andthe tangent to the convex, or predominantly convex, profile at the pointwhere it joins the pipe is at least equal to 5°, and, preferably, lessthan 45°.
 3. Jet spouted bed reactor according to the preceding claim,the angle β being at least equal to 10°, and, preferably, less than 35°.4. Jet spouted bed reactor according to claim 1, the convex orpredominantly convex profile having a point of inflexion, and the angleθ₁ between, on the one hand, an axis perpendicular to the direction offlow of a fluid and extending through the base of the cylindrical partand, on the other hand, the tangent to the point of inflexion I of theconvex profile, being greater than 10°.
 5. Jet spouted bed reactoraccording to claim 1, wherein the angle θ₂ between, on the one hand, anaxis perpendicular to the direction of flow of a fluid and extendingthrough the base of the cylindrical part and, on the other hand, astraight line extending through the end of the gas injection pipe andthrough the base of the cylindrical part, is less than 90°.
 6. Jetspouted bed reactor according to claim 1, wherein the distance (h1)between the end of the gas injection pipe and the base of thecylindrical section of the reactor is greater than 1/10 of the radius ofthe bed in this cylindrical section.
 7. Jet spouted bed reactoraccording to claim 1, wherein the convex or predominantly convex profileis defined by a succession of N (N≧2) straight line segments.
 8. Methodof deposition on particles, comprising: the introduction of saidparticles into a jet spouted bed reactor according to claim 1, theintroduction, via the gas injection pipe, of a gas transporting aprecursor of the deposition to be carried out on said particles. 9.Method according to claim 8, wherein r₁, the radius of the injectionpipe, is comprised between 5 D (D=diameter of the particles) and ⅕ r₂,where r₂ is the radius of the bed in the cylindrical section.
 10. Methodaccording to claim 8, the convex profile having a point of inflexion,and the angle θ₁ between, on the one hand, an axis perpendicular to thedirection of flow of a fluid and extending through the base of thecylindrical part and, on the other hand, the tangent to the point ofinflexion I of the convex, or predominantly convex, profile beinggreater than the angle of spontaneous flow of the powder constitutingthe bed to be mixed on an inclined plane.
 11. Method according to claim8, the angle θ₂ between, on the one hand, an axis perpendicular to thedirection of flow of a fluid and extending through the base of thecylindrical part and, on the other hand, a straight line extendingthrough the end of the gas injection pipe and through the base of thecylindrical part, being greater than the angle of spontaneous flow ofthe powder constituting the bed to be mixed on an inclined plane. 12.Method according to claim 8, at least a part of the particles having adiameter of the order of 500 μm and/or the difference in specific massbetween the fluidisation gas and the powder to be fluidised beinggreater than or equal to 5 g/cm³.
 13. Method according to claim 8,wherein a plurality of successive depositions are carried out on saidparticles.
 14. Method according to claim 8, the particles being fuelparticles used in HTR type nuclear reactors.